Notes on the "Bright Spot", the earth's shadow, and other interesting optical phenomena
Last updated August 25, 2005
Note: at present this article is almost devoid of photos, but I'll be adding some in the future!
The "Bright Spot"
In flight, an observer can often see a brilliant "bright spot" around the shadow of his aircraft. As the aircraft's altitude increases, the aircraft's shadow becomes more diffuse and shrinks (in terms of its angular size as seen by the airborne observer) and the bright spot becomes more and more distinct.
Several different factors cause the "bright spot".
Near the antisolar point (i.e. near the point that is exactly opposite the sun), all shadows are hidden by the objects (trees, blades of grass, etc) that are creating them, and the landscape takes on a brilliant, "flat" appearance. No matter how rugged the landscape is, no shadows will be visible at the antisolar point.
Anyone who has hiked at night while wearing a headlamp will
be familiar with this shadow-hiding effect: the ground is lit so uniformly that
it appears to be a completely flat surface and obstacles are difficult to
detect. An additional flashlight held
at waist-level throws strong shadows from any object that protrudes above the
main surface of the ground, and this will likely save the hiker from
stumbling. For an even more dramatic
demonstration of this effect, ride a bike at night on a potholed gravel road
with only a headlamp for light--the potholes will be completely invisible!
However I suspect that the shadow-hiding effect is not main
phenomenon responsible for the very well-defined, very intense "bright
spot" that can often be observed very close to the antisolar point during
flight. A clue is offered by the fact
that whenever reflective road signs pass through the "bright spot",
they shine brilliantly. I suspect that
the concentrated, intense "bright spot" that can often be observed
around the antisolar point is mainly caused by the reflective properties of
small particles on the outer surfaces of plants. There appears to be a strongly defined region around the
anti-solar point within which the amount of light reflected back to an observer
is much greater than it is in the surrounding regions slightly further from the
I've noticed that the "bright spot" is
particularly brilliant when the antisolar point falls upon sagebrush. Tall grass can also create a rather intense
"bright spot", and conifers create a more intense "bright
spot" than do deciduous trees.
I've not seen a highly-defined "bright spot" on non-vegetated surfaces.
I have a striking pair of photos taken while hang gliding over sagebrush near sunset. In the first photo, the shadow of my upper body and head falls on the undersurface of the wing of the banked glider. In the next photo, the glider has turned slightly, so that the shadow of my head has moved off of the undersurface of the wing and is no longer visible, but the shadow of my upper body on the undersurface of the wing still shows where my head should be. On the distant landscape, right where my head should be, the "bright spot" is shining brilliantly. (The camera was held up to my eyes to take both of these pictures). Near the bright spot, the perfectly round disk of the near-full moon has just cleared the horizon. The dark band of the earth's shadow is also visible in the photo.
An observer casting a strong shadow on the nearby ground cannot see the brightest part of the "bright spot" because his own shadow hides it. However he will often note that the landscape is brighter near the shadow of his head--i.e. directly opposite the sun, in relation to his own eyes--than anywhere else. He will also notice that the shadows created by the texture of the landscape are not visible in the region near the shadow of his head.
An observer standing on ridgeline that is casting a shadow on distant terrain will notice a bright glow at the point on the shadow of the ridgeline that corresponds to his own location.
From the air, the bright spot is often so strikingly bright and well-defined that some observers believe that the edges of the aircraft are somehow focusing the sun's rays to create the bright spot on the ground. I don't subscribe to this theory, nor do I believe that an observer on the ground, standing in the bright spot, would notice any increase in the amount of light striking the ground around him as the aircraft passed across the sun. The strongest objection to this "edge-focussing" theory is the fact that when two aircraft fly in formation, the pilot of each aircraft sees only one "bright spot", centered around the shadow of his own aircraft--i.e. centered around the point directly opposite the sun in relation to his own eyes.
The texture of the landscape affects the shape of the bright spot. For example, the bright spot as seen on a grassy field has a tall, skinny shape. We could interpret this either as being related to the shape of the shadow cast by each blade of grass, or as being related to the directions of near-optimal reflection from each blade of grass.
In this particular photo, the distant landscape near the horizon is brighter than the foreground, because the vertical stalks of grass on the horizon are more optimally positioned to reflect light directly back to the observer than are the vertical stalks of grass in the foreground. Nonetheless a trace of a bright glow is visible in the immediate vicinity of the shadow of the observer's head, where the shadow of each blade of grass vanishes.
The earth's shadow
An observer can often see a dark horizontal
band rise into the eastern sky at sunset and sink into the western sky at
sunrise. The top of the band will often
be tinged with pink or rose. This band
is the earth's shadow, and a person in an aircraft positioned at the top of the
band, in the colorful area, would see a red or orange sun about to sink below
the horizon or just rising from the horizon.
Radiating and converging shadows
When mountains or tall
cumulus clouds lay near the horizon or beyond the horizon, in the direction of
the rising or setting sun, an observer can often see dark shadow-lines
radiating out from the position of the sun, even when the sun itself is below
the horizon. Since parallel lines
converge, under the right viewing conditions these shadow lines can be seen to extend most
of the way across the sky and converge together again as they approach the anti
solar point. Note that angular
elevation from the sun, if it were visible to the viewer, would be lower than
the tops of the mountains or clouds that are casting these shadows. The radiating and converging shadow
phenomenon can occur regardless of whether the sun is above or below the
horizon, and if the sun is below the horizon, the tops of the mountains or
clouds that are casting the shadows may be either above or below the horizon.
Pyramidal mountain shadows
When standing on a mountain
shortly after sunrise or shortly before sunset, an observer can see the shadow
of the mountain. The lower part of the
shadow will be cast on the distant landscape but immediately after sunrise or
immediately before sunset, the upper part of the shadow will be cast in the
sky. An observer in an aircraft at the
top of the shadow would see the sun just barely emerging from the mountain
peak. Due to the fact that all of the
lines of shadow converge at the antisolar point, the shadow as seen by an
observer on the mountain peak always looks like a pyramid shape, even if the
mountain is shaped very differently.
The curvature of the earth is the reason that the antisolar point can be
above the observer's horizon, even though the sun is also above the observer's
I had an interesting experience when I was flying an
airplane about four thousand feet above the top of Oregon's Mt Jefferson,
several miles west of the peak, near sunrise.
It was August and the atmosphere was thick with smoke from distant
forest fires. When I was directly in
line with the peak, so that the sun floated just over the top of the peak, the
mountain's pyramidal shadow floated in the western sky. But when I flew northward a bit, so that the
sun floated to the north of the peak, the shadow in the western sky transformed
into a much darker diagonal ray that slanted through the sky from left to right
(as described from bottom to top). When
I flew back south, the shadow lightened again and formed into a pyramid as the
sun passed over the mountain peak, and when I flew further south the shadow
darkened again and transformed into another diagonal ray, now slanting in the
opposite direction, as the sun passed to the south of the peak. The geometry involved must have been as
follows: since the top of the mountain was at a lower angular elevation (as seen
from my viewpoint) than the sun was, it cast an "upside down" version
of the dark radiating-and-converging shadow beams that we described above. For example if there had been two
mountains--one on each side of the sun--then in the western sky I would have
seen two diagonal lines, slanting in opposite directions, far apart from each
other at their lower portions and nearly converging together at their tops, and
their tops (representing the shadows of the tops of the mountains) would have
been just slightly to the left and the right of (and slightly below) the antisolar point. Also, if the atmosphere had been dusty or
hazy enough, I would have been able to trace each of these shadow formations
right back to the mountain that created it: it would have been obvious that the
shadow lines were radiating out from vicinity of the sun, and converging again
around the antisolar point.
Planets in broad daylight
It's fun to look for planets in broad day
light. Venus and Jupiter can both be
spotted in broad daylight under the right conditions. This is easiest in the clear air of high altitudes. This is also easiest when you the planet's
position in relation to the waning crescent moon, because you spotted the
planet and the moon in the predawn sky before sunrise. To be seen in broad daylight, a planets has to be fairly far from the
sun or it will be lost in the glare of the sun. Polarized sunglasses help greatly, by darkening the sky.
The green rim
I've never seen an actual green
"flash" but I've very often see the uppermost rim of the sun exhibit
an intense emerald green color. This is
most easily seen in the last few seconds before sunset, when the main body of
the sun is below the horizon and the part that remains above the horizon is
shining through the thickest part of the atmosphere, so that it is dimmed
enough for comfortable viewing. Often
this green rim is invisible to the naked eye but easily visible through
binoculars. The green rim is created by
the way that the atmosphere acts as a lens and separates the green, blue, and
indigo wavelengths away from the red, orange, and yellow wavelengths, so that
the former are the last to be seen as the sun slips below the horizon. The dust in the atmosphere scatters away the
blue and indigo colors, so that the green color of the uppermost rim is green,
although in regions of very pure air the rim can turn blue rather than
To see the green rim, a very distant horizon is usually
needed, because the lowermost, thickest part of the atmosphere has the
strongest "lensing" effect, and because when the sun slips behind a
high mountain horizon, it will still be too blinding at the moment of sunset to
allow the green rim to be discerned.
The green rim can also be seen when the moon sets and when
bright planets set--in the latter case, as viewed through binoculars, an actual
rim will not be identifiable but the planet will transform into a tiny green
point of light the instant before it disappears. The green rim can also be seen when objects rise, if the observer
knows exactly where the object will rise and watches that spot with
binoculars. (However the atmosphere is
often clear in the morning than at night, so the sun is more likely to be too
blinding too look at in the morning than at night. And a word of caution: NEVER look at the sun with the naked eye
or binoculars unless it is close enough to the horizon to be very much dimmed
from its usual intensity.)
Just as the uppermost part of the rim of the sun turns
green, so too does the lowermost part of the rim of the sun turn red. This effect is harder to see than the green
upper rim, because when the lowermost part of the rim is visible the main body
of the sun is often high enough in the atmosphere to be too blinding to look
at. However, if the atmosphere contains
enough dust or smoke or haze, so that the sun is not too blinding to look at
even when it is entirely above the horizon, the sun's lower red rim can often
be seen through binoculars at sunrise or sunset. Also, since the moon is much less blinding than the sun, the
moon's lowermost rim can often be seen to be very red for up to 10 minutes or
more after moonrise or before moonset.
Collapsing bubbles and paper lanterns
Another phenomenon that can often be
seen with binoculars as the sun sets is that the edges of the sun take on a
slightly serrated or "zig-zag" appearance, and at the extreme upper
rim of the sun these zig-zags actually come together in such a way that a small
"bubble" of the sun will actually detach from the main body and
collapse into nothingness. These
"bubbles" will often be rimmed with emerald green, and will collapse
into a single point of intense emerald the instant before they vanish. The same phenomenon can be seen with the
moon--in fact since the moon's circumference has a smaller radius of curvature
than the sun's circumference, the effect is even more pronounced with the
moon. Since the moon is not as blinding
as the sun, it is often the case that the same phenomenon can be discerned at the
extreme lower rim of the moon; these bubbles are tinged in red. If the air contains enough dust or smoke or
haze, so that the sun is not too blinding to look at even when it is entirely
above the horizon, the detaching bubbles with their red rims can sometimes be
seen at the lower edge of the sun as well.
The formation of zig-zag serrations in the rim of the sun or
moon, and the associated detaching bubbles, sometimes can be clearly seen to
correlate with the tops of smoke layers or haze layers, which mark inversions
in the atmosphere. Since the moon is
not as blinding as the sun, and since the moon has a lower radius of curvature
than the sun, with the moon this phenomenon can often be discerned as long as
15 to 20 minutes or more after the moonrise or before the moonset, when the
moon is many diameters above the horizon.
A careful watch of the rising or setting moon will often suggest that
there are many dozens of fine-scale temperature inversions in the vertical
structure of the lower atmosphere during stable, high-pressure weather.
When the sun or moon is very near the
horizon, sometimes the zig-zag serrations in the rim become so extreme that the
body takes on a "paper lantern" shape or distorts into an even more
bizarre shape that looks nothing at all like a round disk.
Here are some interesting things that you can observe while
looking at contrails:
* Sometimes a
contrail leaves a shadow in the sky that can be seen by an observer on the
ground. Usually this happens when an
airplane flies over a thin sheet of translucent cirrostratus cloud and the
contrail casts a shadow on the cloud.
* On a cloudless day, a contrail can cast a visible dark
shadow though the air column, extending all the way from the contrail to the
ground. When you--as an observer on the
ground--see a contrail cast a visible dark shadow through the air column, take
a glance toward the sun and you'll invariably find that the contrail passes
directly through the sun. Here's why:
imagine that the shadow of the contrail is like a sheet of (reflection-free)
glass with a very slight tint. If you
look through the pane of glass perpendicularly (as if looking through a
window), the pane of glass will be invisible and you will have an unobstructed
view of what lies beyond. If the tint
is very slight, the glass will remain invisible even when viewed at a very
shallow angle rather than a perpendicular, 90-degree angle. But if you try to look through the pane of
glass in a true edge-wise fashion, the pane will look like an opaque line. The shadow of a (linear) contrail through
the air column is shaped like a flat pane of glass, miles long and tens of
thousands of feet tall and only a hundred feet or less in width. And when the contrail happens to pass across
the face of the sun, the geometry is such that you are looking at the edge
rather than the face of the contrail's shadow.
That's why the shadow becomes visible to you, even though the contrail
is only shadowing a very thin slice of a big, bright, luminous sky. This geometry also causes the dark line of
the contrail's shadow to project directly forward from the aircraft, as seen by
the observer on the ground. This gives the strange illusion that "the shadow
knows" the course that the airplane will follow. This photo illustrates such a shadow: there are many contrail segments in the photo, but only the segment which, when extended, will pass directly through the sun as seem by the observer, leaves a dark shadow in the airmass that is easily visible to the observer.
* Of course, contrails that linger for many minutes or tens
of minutes indicate a high moisture content in the upper atmosphere and signal
an imminent increase in high cloud cover.
In some cases the contrails themselves can contribute significantly to
the increase in high cloud cover, or perhaps more commonly, cause the increase
in high cloud cover to happen several hours earlier than it otherwise would
* The above facts, along with the fact that high, thin,
translucent clouds (including contrails) often have an iridescent sheen
(especially when viewed through polarized sunglasses), have actually led to the
"chemtrail" conspiracy theory! The sudden popularity of this conspiracy theory--which is based
on the assertion that some sort of bizarre, new phenomenon is happening with
contrails-- illustrates that a lot of people have spent most of their lives in
a remarkably unobservant state.
* Under certain meteorological conditions--typically
accompanied by patchy, thin, cirrostratus cloud cover--an observer with
binoculars can see a contrail sheeting off the entire wingspan of a high-flying
aircraft, rather than forming behind each engine. This is a vivid illustration of the pressure drop that occurs
immediately above a wing. However this
phenomenon is rare--I've been paying attention to the sky for all my life, and
I've only seen it a handful of times.
* Related phenomena: the visible vapor trails that stream
from the wingtips of fighter aircraft that are "pulling G's" in humid
conditions, or from the wingtips of any aircraft flying at a high
angle-of-attack (such as a heavily loaded aircraft flying slowly) in humid
conditions. Also the vapor trails that
stream from gaps between the aileron and the rest of the wing during hard
aerobatic maneuvering. In all these
cases the vapor trail is forming in the low-pressure core of an intense
* If you look with binoculars at an airplane with one engine
on each wing, the contrails often appear to be streaming from the tips of the
vertical stabilizer. Of course this is
not really the case--it's just that the exhaust doesn't cool enough for the
water vapor to condense until it has travelled a few tens of feet away from the
* If you look with binoculars at an airplane with two engines
on each wing, the two contrails forming behind each wing usually twist around
each other one time (so that the outboard trail switches place with the inboard
trail) and then coalesce into a single, well-defined trail. As the contrails are braiding together and
coalescing, they are also drifting outboard from the centerline of the
aircraft. So a four-engined airplane
usually has two well-defined contrails streaming behind it, with a distance of
about one to two wingspans between them.
These contrails undoubtedly mark the wingtip vortices. 747's in particular produce a very
distinctive contrail "signature": the 4 contrails make a very
characteristics shape as they drift outboard and twist around each other and
coalesce into 2 well-defined trails, which maintain their tight, defined shape
for longer than do the contrails from most other aircraft.
* Here are some more things you can often see while looking
at contrails: ice crystals sheeting downward from the contrail. The contrail abruptly starting and/or
stopping as the aircraft passes through different temperatures and/or moisture
levels (which are often also marked by patches of high cloud). The contrail taking a sinuous, twisting
shape as it is sheared by differences in wind speed.
* Of course, in a uniform airmass, a contrail marks an
aircraft's direction of travel through the airmass, not the aircraft's
direction of travel in relation to the ground.